How to: Processing the Master Bus

In a mix’s signal flow, the master bus is the final point before your audio gets summed and rendered to a stereo file. It’s the only chance during the mix to apply processing across all of your tracks together.

Master bus inserts give you the opportunity to apply processing to all the tracks at once, to add a little polish to your mix.

It’s an opportunity to make subtle but significant tweaks to the EQ of your song, to “glue” the mix together with compression and maybe even add a little tape or tube saturation to give it some analog vibe.

Although there are still people who use hardware-based master-bus processing chains, many of us mix in the box using plug-ins. These days, the software emulations of hardware processors are so good that the differences between them and the original units are less consequential.

If you’re working in the box and using quality plug-ins, you can feel pretty confident that you’re getting a reasonable facsimile of the performance someone would get using hardware, with a lot more convenience.

WHEN TO ADD BUS PROCESSING

You might assume that everyone does their master-bus processing as the last step in the mixing process. But if you talk to pro engineers, you’ll find that there are actually two schools of thought about that.

The more conventional approach is to add the processing at the last stage of the mix. The idea is that at that juncture, you can better judge what your mix is lacking and make any global corrections based on that. Does it need a little more “air” (high-end EQ)? Perhaps some more low-end punch? How do I make it sound more like the mix reference I’m using? Does it need some light compression to help integrate the tracks into a more consistent whole? There’s definitely nothing wrong with this approach, which is almost like “pre-mastering.”

The other school of thought says it is better to apply bus processing, especially compression, at the beginning and “mix into it.” These engineers feel that is a more organic way to go about it because all of your mix decisions are made in the context of that processing. And, of course, you can always adjust it at the end if need be.

Whichever route you take, virtually everyone agrees that master bus processing needs to be subtle, because it has the potential to really detract from your mix.

PROCESS OR NOT?

Some people advocate that you don’t do any mix-bus processing at all. They say that rather than trying to fix problems globally, it’s better to address the individual tracks that are causing the issues. Not surprisingly, that’s an opinion that you’ll hear from mastering engineers who often have to spend time trying to undo heavy-handed master-bus processing.

Whether your mix will go to a mastering engineer or whether you’re self-mastering is relevant to how much you process the master bus. If you’re going to spend a lot of money to get your music professionally mastered, it makes sense to leave much of the global processing to the mastering engineer. After all, when it comes to processing songs that have already been mixed, those engineers have a ton of expertise. The exception would be any processing used in service of creative ideas, such as compressing heavily to create “attitude” in the sound.

Some mastering engineers recommend that you send them two versions of each mix, one with your master-bus processing and one without. That makes sense because it allows them to hear what you are going for with the processing but gives them room to maneuver. It also allows you to create a “loud” mix to send for mix approval. If you know before the mix session who you’ll be using for mastering, that’s a good time to touch base with that person about the issue of master-bus processing.

ALL THINGS BEING EQUALIZED

EQ is usually added on the master bus for one of a few different reasons. As mentioned previously, many engineers like to add a small boost in the high end to provide some extra “air,” which can add clarity and sheen and help a mix sound more finished.

The frequency boosted varies quite a bit, but typically, you would use a shelving filter set between 8 to 12 kHz, so everything above the selected frequency would get a boost (see Fig. 1). Because a lot of muddiness can occur in the lower-midrange, it’s not uncommon to also try a slight cut in the neighborhood of 300 to 500 Hz to address that.

Fig. 1: A shelving filter, like this one shown in iZotope Ozone 8, is typically used when adding high-end “air” to the mix.

With any master bus EQing, you want to keep your boosts and cuts subtle, in the range of 1 to 2 dB. If your mix needs more than that, you’re better off addressing the frequencies of the individual tracks to provide the tonality you’re looking for.

Some engineers use a highpass filter set around 30 Hz to remove subsonic frequencies that aren’t needed. You probably wouldn’t want to do this if you’re working in a genre like dance music or hip-hop where those frequencies are essential, but for other types of music it can help clean up the bottom end.

However, there is definitely an argument to be made that you’re better off doing any highpass filtering on the individual tracks rather than the master, because on most mix elements you can get away with setting the frequency a lot higher than 30 Hz. If you’re going for global highpass filtering, however, don’t set it above 30 Hz or you’ll take away bottom from the bass and kick.

If you’re going to EQ and compress the master bus, there’s no hard-and-fast rule as to which one to put first in the signal chain. But if you do have the EQ first, be aware that your EQ settings may be affect the response of the compressor because low frequencies trigger compression more heavily than mid-and high-frequency audio does.

COMPRESS FOR SUCCESS

Mixing is the art of taking the individual tracks that make up a multitrack recording and blending them into stereo (or other playback format) in service of the song. Depending on the genre and song, you could be aiming for energy, excitement, rawness, smoothness, an organic and natural feel or any number of attributes. But in virtually any mix, cohesion is critical.

You want the mix to sound like an organic whole, rather than a bunch of disparate elements artificially squashed together. One of the main reasons to apply a compressor across the master bus is to help provide more cohesiveness, which is also referred to as “glue.” Not only does a compressor impart a similar sound on every track, especially if it adds tonal color, it globally applies dynamic control.

Which type of compressor to use is a matter of taste. You could go with an emulation of a console master-bus compressor, such as the SSL G-Series bus-compressor emulations offered by Waves, UAD and others. The SSL bus compressors used VCA-based designs, but all different compressor types have been used on the stereo bus including optical compressors and tube compressors.

If you’re going for glue, you want to compress pretty lightly, with a low ratio (1.5:1 or 2:1) and gain reduction of only about 1 to 2 dB. The attack and release settings are crucial. The faster the attack, the more the initial transients will be attenuated. If you set it too fast, the results can sound unnatural and lifeless, so be careful. If you’re going for transparency and for glue, keep the attack settings on the slower side (see Fig. 2).

Fig. 2: This shows the Fab-Filter Pro C2 compressor being used on a master bus. Note the slow attack time and small amount of gain reduction.

The release is a bit trickier. The release setting governs how long the compressor holds after it’s triggered. Faster releases will tend to feel more energetic while slower ones will offer smoothness. Experiment with different settings as the song plays and try to find what feels right. If your compressor has an auto-release function, you can try that, too, and see if it yields a good result. If your compressor comes with master-bus presets, they’re often a good place to start. Just don’t be afraid to adapt the parameters to fit your song.

The previous advice about master bus compression was based on the conventional approach of applying it subtly. On the other hand, if you want to slam your track with heavy compression, and it works for what you’re going for, there is no rule saying you can’t do it. In fact, there are plenty of engineers who use fairly robust settings on their master bus compressors for certain types of music.

OVER THE LIMIT

The so-called “loudness wars,” in which everyone tried to make sure their songs were loud enough that they would hold up in comparison to others played before and after, are basically over. Streaming music is the most common delivery format now, and all the major streaming services have adopted similar loudness limits. It’s no longer critical to make your song loud to compete with others, because everything gets evened out in volume by the processor of the streaming services. What’s more, CDs, which are generally mastered at louder overall levels than music destined for streaming, are in decline.

As you’re probably aware, over-use of a limiter in an attempt to create loudness can reduce your song’s dynamic range, so limiting is something you should be extremely judicious with if you use it at all. It’s probably only worth doing if you’re self-mastering, and, even so, it might be better to apply it later during a dedicated mastering session, especially if you’re working on a multi-song project where you have to match the volumes of the various songs.

If you are going to use a limiter, go easy and be careful not to introduce distortion. Sometimes when you’re comparing the sound with and without the limiter, the jump in volume from the limiter creates the louder-is-better effect. That can fool you into thinking you’ve improved the sound and make you less cognizant of possible distortion introduced by the limiter.

If you can, try to compare the limited and non-limited audio at the same level. That makes it easier to evaluate what the process is doing to the signal beyond just making it loud. One way to do this is to use your monitor volume control to compensate and turn it down when you turn the effect on and back up when you bypass the limiter. Plugins such as iZotope Ozone 8 offer a helpful gain-matching feature that does this automatically.

SATURATION

Today’s plug-in environment is positively teeming with processors that provide some sort of harmonic distortion, including emulations of tape, tube and transistor gear. Even more than other types of master-bus processing, the operative word with saturation is “restraint.” In most cases, you probably don’t want it to be an obvious effect. Instead, it should sound organic to the mix, as if you were mixing on gear that imparted natural saturation.

Tape-emulation plug-ins such as the UAD ATR-102, Waves J37, Softube Tape and others can be inserted on the master bus to add tape characteristics. Console plug-ins, such as the Slate Virtual MixBuss (a component in the Virtual Console Collection) add attributes of analog mixing consoles including transformer saturation. Other options include tube emulation such as Wave Arts Tube Saturator or a hybrid plug-in with a variety of saturation choices such as FabFilter Saturn.

There are a couple of issues to consider. First, some saturation plug-ins are designed to be used primarily on individual instruments and might be a little bit low-fi for the master bus—even with the distortion only subtly applied. One way to get a good sense for what a plugin is intended for is to look at its presets. If there aren’t master bus or mastering presets, and everything is for individual instruments, you might want to avoid putting them on the stereo bus. But, naturally, your ear is the final judge.

I talked earlier about the somewhat controversial topic of whether to add mix-bus processing at the beginning or end of the process. Saturation could be a good candidate for the former approach. Rather than trying to graft it on at the end, if you’re listening to a subtle tape emulation the whole way through your mix, it can cause you to make decisions that integrate better with its sound.

On the flip side, you could also use it successfully in mastering, and not put it on at all during the mix. The possibilities are open ended, and a lot of it comes down to how comfortable you are doing it one way or another, and what works best with your music. When it comes to mixing, rules are meant to be broken—especially if you can get good results by doing so.

THE LAST BUS

When artfully applied, master-bus processing can make a significant difference sonically. It is not going to fix a bad mix, but it can certainly enhance a good one. Think of it like icing on a cake.

The more often you employ processing on the master bus, the better you’ll get at it. So if you’re mixing a project without imminent deadlines, try experimenting with different types of processing to increase your skill set and train your ears. Knowing what effects to use, how much and when can only be learned through experience.